1. Technical Field
The present invention relates to a rotor, a vibration motor having the same, and a fabrication method of thereof.
2. Description of the Related Art
The vibration motor having an eccentric rotor is currently widely used in mobile phones and PDAs, etc., as a means of creating vibration upon receipt of incoming calls. As telecommunication devices decrease in size, the demand for smaller and thinner vibration motors is also increasing.
FIG. 1 is a cross-sectional view illustrating the structure of a conventional vibration motor. The conventional vibration motor has a bracket 1 at the bottom. One end of a shaft 9 is inserted and secured to the center of the bracket 1, and the other end of the shaft 9 is secured by a case 8. The case 8 protects the other parts of the vibration motor from external interferences. A flexible board 12 of minute thickness is placed on the top of the bracket 1.
A multi-polar magnet 2 with alternating N, S poles along its circumference is placed on the perimeter of the central flexible board 12, and in the central cavity of the magnet 2, a pair of brushes 3 are positioned at a designated angle with the lower ends attached to the flexible board 12. A bearing 11 is inserted onto a designated position of the shaft 9, and an eccentric rotor 10 is inserted onto the perimeter of the bearing 11. A plurality of commutators 7 that are in contact with the brushes 3 are positioned on the bottom of the rotor 10.
FIG. 2a is a perspective view illustrating the upper portion of the conventional eccentric rotor 10.
As shown in FIG. 2a, the rotor 10 is equipped with a board 4 that is cut from a flat circular plate, and a plurality of wound coils 5 arranged in designated angles are positioned on the upper surface of the board 4. Also, a weight 13 that increases the eccentricity of the rotor 10 is located on the board 4 between the wound coils 5. The wound coils 5 and weight 13 are secured to the board 4 by, for example, a molded form 6 made of a material such as plastic, etc.
FIG. 2b is a perspective view illustrating the bottom of the conventional eccentric rotor 10. As shown in FIG. 2b, the commutators 7 of a flat plate shape are arranged radially around the rotation axis of the rotor 10 on the bottom of the board 4.
In such a vibration motor, the rotor 10 is rotated by the electromagnetic interaction between the wound coils 5 and magnet 2, as electric currents from an external source are supplied through the flexible board 12 and brushes 3 to the wound coils 5. The rotor 10 is operated eccentrically, as it is eccentrically supported by the shaft 9 of which both ends are secured by the bracket 1 and case 8, respectively. This eccentric driving power is transferred via the shaft 9 to the bracket 1, resulting in vibration.
Therefore, it can be seen that the vibration effect of the vibration motor occurs due to the eccentricity of the rotor 10 from the disproportionate concentration of mass caused by the weight 13, etc. Consequently, it is required to increase the eccentricity of the rotor 10 to obtain a greater vibration.
As described above, wound coils 5 are used in a conventional rotor 10, but the wound coils 5 require increased fabrication time and cost. In addition, the wound coils 5 are substantial in volume, causing an increase in volume of the rotor 10 and thus the vibration motor. Furthermore, the coils 5 are generally very thin, of about 45-55 μm, so that the coils 5 often snap during the fabrication process, thus incurring increased defect rate.
Also, as the wound coils 5 must be accurately attached at constant intervals from the center of the board 4, the accurate positioning and attaching of the wound coils 5 cause the problems of increased fabrication time and cost.
In addition, the weight 13 is formed on the board 4 within a limited space, but since the weight 13 is positioned together with the wound coils 5, there is difficulty in increasing the size of the weight 13. In particular, when the size of the weight 13 is increased so as to provide a greater eccentricity of the rotor 10, the size of the wound coils 5 is decreased, causing a reduction in the amount of vibration of the rotor 10. Thus, as seen in FIG. 2a, the rotor 10 is positioned between the wound coils 5 in a limited size, and given a rotor size, there is a limit as to how eccentric the rotor 10 can be made.